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Sökning: WFRF:(Pullerits Tönu)

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1.
  • Alvarez, Sol Laura Gutierrez, et al. (författare)
  • Morphology-Dependent One- and Two-Photon Absorption Properties in Blue Emitting CsPbBr3Nanocrystals
  • 2022
  • Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 13:22, s. 4897-4904
  • Tidskriftsartikel (refereegranskat)abstract
    • The linear and nonlinear optical parameters and morphologic dependence of CsPbBr3 nanocrystals (NCs) are crucial for device engineering. In particular, such information in asymmetric nanocrystals is still insufficient. We characterized the OPLA (σ1) and TPA cross sections (σ2) of a series CsPbBr3 nanocrystals with various aspect ratios (AR) using femtosecond transient absorption spectroscopy (TAS). The σ1 presents a linear volume dependence of all the samples, which agrees with the previous behavior in CsPbBr3 QDs. However, the σ2 values do not exhibit conventional power dependency of the crystal volume but are also modulated by the shape-dependent local field factors. In addition, the local field effect in CsPbBr3 NCs is contributed by their asymmetric morphologies and polar ionic lattices, which is more pronounced than in conventional semiconductor NCs. Finally, we revealed that the lifetimes of photogenerated multiexcitonic species of those nanocrystals feature identical morphology independence in both OPLA and TPA.
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2.
  • Bamini, Sesha, et al. (författare)
  • Time-resolved terahertz spectroscopy reveals the influence of charged sensitizing quantum dots on the electron dynamics in ZnO
  • 2017
  • Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 19:8, s. 6006-6012
  • Tidskriftsartikel (refereegranskat)abstract
    • Photoinitiated charge carrier dynamics in ZnO nanoparticles sensitized by CdSe quantum dots is studied using transient absorption spectroscopy and time-resolved terahertz spectroscopy. The evolution of the transient spectra shows that electron injection occurs in a two-step process, where the formation of a charge transfer state (occurring in several picoseconds) is followed by its dissociation within tens of picoseconds. The photoconductivity of electrons injected into the ZnO nanoparticles is lower than that of charges photogenerated directly in ZnO. We conclude that the motion of injected electrons in ZnO nanoparticles is strongly influenced by their interaction with positive charges left in the sensitizing quantum dots.
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3.
  • Beenken, Wichard, et al. (författare)
  • Excitonic coupling in polythiophenes: Comparison of different calculation methods
  • 2004
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 120:5, s. 2490-2495
  • Tidskriftsartikel (refereegranskat)abstract
    • In conjugated polymers the optical excitation energy transfer is usually described as Forster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers. (C) 2004 American Institute of Physics.
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4.
  • Beenken, W J D, et al. (författare)
  • Excited state properties of neutral and charged ter-fluorene with and without a keto-defect
  • 2008
  • Ingår i: Physica Status Solidi. B: Basic Research. - : Wiley. - 0370-1972. ; 245:5, s. 849-853
  • Tidskriftsartikel (refereegranskat)abstract
    • Stimulated by the recent experimental report of optical properties of charged oligofluorenes [S. Fratiloiu et al., J. Phys. Chem. B 210, 59845993 (2006)], we have used time-dependent density functional theory (TD-DFT) to study the electronic structure of neutral and charged ter-fluorene with and without keto-defect. We have characterized the excited states by site- and space-based representations of the density matrix. For the defectless neutral species we found typical excitons, while the first excited states of the corresponding charged species represent long-range oscillations of the electron or hole along the whole oligomer. For the neutral ketonized ter-fluorene we found trapping of the excited electron. Similarly, the unpaired electron of the anion ground' state is trapped. The excited state properties of the cation; however, are insensitive to the keto-defect.
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5.
  • Beenken, Wichard, et al. (författare)
  • Potential surfaces and delocalization of excitons in dimers
  • 2002
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 117:12, s. 5810-5820
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present work we will demonstrate that the nuclear dynamics have a strong influence on the delocalization of an exciton in a dimer, even if they do not effect the excitonic interaction. It will be shown that the internal nuclear conformation of the molecules forming the dimer depends critically on the delocalization of the exciton state in the dimer and vice versa. The resulting closed loop enforces a localization of the lower excitonic state, but, contrary to the commonly accepted view, a delocalization of the upper one. Qualitatively different time-evolution of the delocalization length for the lower and upper excitonic state will be shown. Besides, it will turn out that the nuclear motions inhibit a complete delocalization of the excitonic state in any case. To accomplish nuclear and exciton dynamics, the nonadiabatic coupling between the two excitonic states will be deduced. This causes a relaxation from the upper to the lower excitonic state, which limits the maximum reachable exciton delocalization.
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6.
  • Beenken, Wichard, et al. (författare)
  • Spectroscopic units in conjugated polymers: A quantum chemically founded concept?
  • 2004
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:20, s. 6164-6169
  • Tidskriftsartikel (refereegranskat)abstract
    • In conjugated polymers the concept of spectroscopic units belonging to different spatial segments of the chain, which are responsible for the spectroscopic properties of the polymer, has been used to explain the spectral heterogeneity and the excitation migration by (Forster type) hopping transfer. In the present work we study the possible mechanism of segmentation of polythiophene into spectroscopic units by using quantum-chemical methods (ZINDO). We found that static geometric defects such as kinks or torsions do not result in a significant localization of the excited states to a certain segment. Hence, we propose that a dynamic localization of excitation due to the interaction between the nuclear and electronic degrees of freedom is responsible for the formation of the spectroscopic units.
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7.
  • Benazzi, Elisabetta, et al. (författare)
  • Acid-triggering of light-induced charge-separation in hybrid organic/inorganic molecular photoactive dyads for harnessing solar energy
  • 2021
  • Ingår i: Inorganic Chemistry Frontiers. - : Royal Society of Chemistry (RSC). - 2052-1553. ; 8:6, s. 1610-1618
  • Tidskriftsartikel (refereegranskat)abstract
    • H+ modulated charge-transfer in photoexcited covalent polyoxometalate-bodipy conjugates is described. The hybrid organic/inorganic molecular photoactive dyads are based on Keggin-type polyoxometalates (POMs, where KM = [PM11O39] and M = Mo or W) covalently grafted via an organotin linker to a bodipy (BOD) photosensitizer. The relative potentials of the photosensitizer and POM are aligned such that light-induced electron transfer from BOD to POM is permitted for the polyoxomolybdate KMoSn[BOD] but not effective for the polyoxotungstate analogue KWSn[BOD]. In both cases, the addition of acid shifts the redox potential of the POM only, to increase the driving force for electron transfer. This leads to charge-separation being switched on for KWSn[BOD] in the presence of acid. The addition of acid to KMoSn[BOD] accelerates charge-separation by an order of magnitude (from 2 ns to 200 ps) and is accompanied by a deceleration of charge recombination, leading to a charge-separated state lifetime of up to 1.3 μs. This behaviour is consistent with proton coupled electron transfer, which has previously been observed electrochemically for POMs, but this study shows, for the first time, the impact of protonation on photoinduced electron transfer. This journal is
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8.
  • Bi, Chenghao, et al. (författare)
  • Spontaneous Self-Assembly of Cesium Lead Halide Perovskite Nanoplatelets into Cuboid Crystals with High Intensity Blue Emission
  • 2019
  • Ingår i: Advanced Science. - : Wiley. - 2198-3844. ; 6:13
  • Tidskriftsartikel (refereegranskat)abstract
    • Colloidal all-inorganic perovskite nanocrystals have gained significant attention as a promising material for both fundamental and applied research due to their excellent emission properties. However, reported photoluminescence quantum yields (PL QYs) of blue-emitting perovskite nanocrystals are rather low, mostly due to the fact that the high energy excitons for such wide bandgap materials are easily captured by interband traps, and then decay nonradiatively. In this work, it is demonstrated how to tackle this issue, performing self-assembly of 2D perovskite nanoplatelets into larger size (≈50 nm × 50 nm × 20 nm) cuboid crystals. In these structures, 2D nanoplatelets being isolated from each other within the cuboidal scaffold by organic ligands constitute multiple quantum wells, where exciton localization on potential disorder sites helps them to bypass nonradiative channels present in other platelets. As a result, the cuboid crystals show an extremely high PL QY of 91% of the emission band centered at 480 nm. Moreover, using the same synthetic method, mixed-anion CsPb(Br/Cl) 3 cuboid crystals with blue emission peaks ranging from 452 to 470 nm, and still high PL QYs in the range of 72–83% are produced.
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9.
  • Bi, Chenghao, et al. (författare)
  • Stable CsPb1- xZn xI3Colloidal Quantum Dots with Ultralow Density of Trap States for High-Performance Solar Cells
  • 2020
  • Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 32:14, s. 6105-6113
  • Tidskriftsartikel (refereegranskat)abstract
    • All inorganic halide perovskites in the form of colloidal quantum dots (QDs) have come into people's view as one of the potential materials for the high-efficiency solar cells; nevertheless, the high surface trap density and poor stability of QDs restrict the performance improvement and application. Here, we obtain colloidal inorganic perovskite CsPb1-xZnxI3 QDs by the hot-injection synthesis process with the addition of ZnCl2. Synchrotron-based X-ray absorption fine structures demonstrate that the guest Zn2+ ions are doped into the CsPbI3 structure to improve the local ordering of the lattice of the perovskite, reducing the octahedral distortions. The increase of the Goldschmidt tolerance factor and the Pb-I bond energy also enhance the stability of the perovskite structure. Furthermore, the Cl- ions from ZnCl2 occupy the iodide vacancies of the perovskite to decrease the nonradiative recombination. The synergistic effect of doping and defect passivation makes for stable colloidal CsPb0.97Zn0.03I3 QDs with ultralow density of trap states. The champion solar cell based on the QDs shows a power conversion efficiency of 14.8% and a largely improved stability under ambient conditions.
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10.
  • Brueggemann, Ben, et al. (författare)
  • Nonperturbative modeling of fifth-order coherent multidimensional spectroscopy in light harvesting antennas
  • 2011
  • Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 13:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent advances in coherent multidimensional spectroscopy have boosted interest in exciton coherences in light harvesting complexes. We present nonperturbative calculations of two-dimensional (2D) electronic spectroscopy from a fifth-order phase-matching direction. The calculations show clear patterns that correspond to the electronic structure of one- and two-exciton manifolds of a Fenna-Matthews-Olsson light harvesting complex. Such signals can provide new information about the coherent properties of antenna pigment protein complexes.
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11.
  • Brueggemann, Ben, et al. (författare)
  • Temperature dependent exciton-exciton annihilation in the LH2 antenna complex
  • 2009
  • Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 357:1-3, s. 140-143
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-color pump-probe measurements of the peripheral light harvesting complex LH2 of Rb.sphaeroides reveal strong temperature dependence of the annihilation rate. The experimental results were modeled via multi-exciton density matrix theory. Based on available literature data we can set an upper limit for the feasible intramolecular internal conversion rate. We show that this also restricts the possible values of the still ill-determined energy of the doubly-excited molecular level of the bacteriochlorophyll, which is responsible for the annihilation process. (C) 2008 Elsevier B.V. All rights reserved.
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12.
  • Brüggemann, Ben, et al. (författare)
  • Control of electron transfer pathways in a dye-sensitized solar cell
  • 2006
  • Ingår i: Physical Review Letters. - 1079-7114. ; 97:20
  • Tidskriftsartikel (refereegranskat)abstract
    • Using shaped laser pulses, we increase the yield of ultrafast electron injection from the sensitizer to TiO2 nanocrystals in the core part of a dye-sensitized solar cell. The temporal structure of the optimized excitation pulse is in clear correlation with nuclear oscillations in the impulsively excited dye molecule. From DFT structure optimization and normal mode analyses we identified the modes which are responsible for the oscillations. The best pulse shape suggests Impulsive Stimulated (anti-Stokes) Raman scattering as a key process of optimization.
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13.
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14.
  • Brüggemann, Ben, et al. (författare)
  • Laser pulse control of exciton dynamics in a biological
  • 2007
  • Ingår i: Journal of Photochemistry and Photobiology, A: Chemistry. - : Elsevier BV. - 1873-2666 .- 1010-6030. ; 190:2-3, s. 372-377
  • Tidskriftsartikel (refereegranskat)abstract
    • Femtosecond laser pulse control of exciton dynamics in a biological chromophore complex is studied theoretically. The computations use the optimal control theory specified to open quantum systems and formulated in the framework of the rotating wave approximation. Based on the laser pulse induced formation of an excitonic wave packet the possibility to localize excitation energy at a certain chromophore within a photosynthetic antenna system (FMO complex of green bacteria) is investigated. Details of exciton dynamics driven by a polarization shaped pulse are discussed. (c) 2007 Elsevier B.V. All rights reserved.
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15.
  • Brüggemann, Ben, et al. (författare)
  • Laser pulse control of exciton dynamics in the FMO complex: Polarization shaping versus effects of structural and energetic disorder
  • 2006
  • Ingår i: Journal of Photochemistry and Photobiology, A: Chemistry. - : Elsevier BV. - 1873-2666 .- 1010-6030. ; 180:3, s. 322-327
  • Tidskriftsartikel (refereegranskat)abstract
    • Femtosecond laser pulse control of exciton dynamics in biological chromophore complexes is studied theoretically using the optimal control theory specified to open quantum systems. Based on the laser pulse induced formation of an excitonic wave packet the possibility to localize excitation energy at a certain chromophore within a photosynthetic antenna system (FMO complex of green bacteria) is investigated both for linearly polarized and polarization shaped pulses. Results are presented for an ensemble of N energetically disordered and randomly oriented FMO complexes. Here, the optimized control pulse represents a compromise with respect to the solution of the control task for any individual complex of the ensemble. For the case of an ensemble with N = 10 members the polarization shaped control pulse leads to a higher control yield compared with a linearly polarized pulse. This difference becomes considerably smaller for an ensemble with N = 120 members. The respective optimized pulses are used to drive excitation energy in a different ensemble with M >> N complexes to simulate the usual experimental condition in solution. For the case with N = 120, the relative control yield coincides with the resulting control yield "in solution", giving a slightly higher control yield for polarization shaped pulses. (c) 2006 Elsevier B.V. All rights reserved.
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16.
  • Brüggemann, Ben, et al. (författare)
  • Non-perturbative calculation of 2D spectra in heterogeneous systems: Exciton relaxation in the FMO complex
  • 2007
  • Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 444:1-3, s. 192-196
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental 2D frequency resolved spectra showing the relaxation of excitons in the photosynthetic antenna system FMO are reproduced by a multi-exciton density matrix approach explicitly including the external fields. The interference of the polarizations of a number of randomly placed, rotated and energetically inhomogencous complexes is used to extract the signal in the phase matching directions. It is shown that varying the laser intensity has only a minor impact, but changing the phase within the single pulses makes higher order contributions possible which lead to a fundamental change of the 2D spectra. (C) 2007 Elsevier B.V. All rights reserved.
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17.
  • Busto, David, et al. (författare)
  • Probing electronic decoherence with high-resolution attosecond photoelectron interferometry
  • 2022
  • Ingår i: European Physical Journal D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 76:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Abstract: Quantum coherence plays a fundamental role in the study and control of ultrafast dynamics in matter. In the case of photoionization, entanglement of the photoelectron with the ion is a well-known source of decoherence when only one of the particles is measured. Here, we investigate decoherence due to entanglement of the radial and angular degrees of freedom of the photoelectron. We study two-photon ionization via the 2s2p autoionizing state in He using high spectral resolution photoelectron interferometry. Combining experiment and theory, we show that the strong dipole coupling of the 2s2p and 2p2 states results in the entanglement of the angular and radial degrees of freedom. This translates, in angle-integrated measurements, into a dynamic loss of coherence during autoionization. Graphic Abstract: [Figure not available: see fulltext.]. © 2022, The Author(s).
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18.
  • Camacho Dejay, Rafael, et al. (författare)
  • Fluorescence polarization measures energy funneling in single light-harvesting antennas-LH2 vs conjugated polymers.
  • 2015
  • Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5
  • Tidskriftsartikel (refereegranskat)abstract
    • Numerous approaches have been proposed to mimic natural photosynthesis using artificial antenna systems, such as conjugated polymers (CPs), dendrimers, and J-aggregates. As a result, there is a need to characterize and compare the excitation energy transfer (EET) properties of various natural and artificial antennas. Here we experimentally show that EET in single antennas can be characterized by 2D polarization imaging using the single funnel approximation. This methodology addresses the ability of an individual antenna to transfer its absorbed energy towards a single pool of emissive states, using a single parameter called energy funneling efficiency (ε). We studied individual peripheral antennas of purple bacteria (LH2) and single CP chains of 20 nm length. As expected from a perfect antenna, LH2s showed funneling efficiencies close to unity. In contrast, CPs showed lower average funneling efficiencies, greatly varying from molecule to molecule. Cyclodextrin insulation of the conjugated backbone improves EET, increasing the fraction of CPs possessing ε = 1. Comparison between LH2s and CPs shows the importance of the protection systems and the protein scaffold of LH2, which keep the chromophores in functional form and at such geometrical arrangement that ensures excellent EET.
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19.
  • Chen, Junsheng, et al. (författare)
  • Two-photon absorption in halide perovskites and their applications
  • 2022
  • Ingår i: Materials Horizons. - : Royal Society of Chemistry (RSC). - 2051-6347 .- 2051-6355.
  • Tidskriftsartikel (refereegranskat)abstract
    • Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.
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20.
  • Chen, Miaoxiang, 1962-, et al. (författare)
  • High carrier mobility in low band gap polymer-based field-effect transistors
  • 2005
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 87:25, s. 252105-1-252105-3
  • Tidskriftsartikel (refereegranskat)abstract
    • A conjugated polymer with a low band gap of 1.21 eV, i.e., absorbing infrared light, is demonstrated as active material in field-effect transistors (FETs). The material consists of alternating fluorene units and low band gap segments with electron donor-acceptor-donor units composed of two electron-donating thiophene rings attached on both sides of a thiadiazolo-quinoxaline electron-acceptor group. The polymer is solution-processable and air-stable; the resulting FETs exhibit typical p-channel characteristics and field-effect mobility of 0.03 cm2 V−1 s−1.
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21.
  • Christensson, Niklas, et al. (författare)
  • Determining Vibrational Huang-Rhys Factors by Photon Echo Spectroscopy
  • 2009
  • Ingår i: Ultrafast Phenomena XVI. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 0172-6218. ; 92, s. 334-336
  • Konferensbidrag (refereegranskat)abstract
    • Electronic and vibrational dephasing dynamics of Rhodamine 800 has been studied with 3PEPS. With careful analysis, the S-factors of the vibrational modes can be accurately determined. The vibrational dephasing rate displays abnormal frequency dependence.
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22.
  • Christensson, Niklas, et al. (författare)
  • Electronic Double-Quantum Coherences and Their Impact on Ultrafast Spectroscopy: The Example of beta-Carotene
  • 2010
  • Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 1:23, s. 3366-3370
  • Tidskriftsartikel (refereegranskat)abstract
    • The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated beta-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy, surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S-2 -> S-n2). The signatures of this transition in pump-probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence.
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23.
  • Christensson, Niklas, et al. (författare)
  • Electronic photon echo spectroscopy and vibrations
  • 2010
  • Ingår i: Vibrational Spectroscopy. - : Elsevier BV. - 0924-2031. ; 53:1, s. 2-5
  • Konferensbidrag (refereegranskat)abstract
    • Coupling between electronic and nuclear degrees of freedom in Rhodamine 800 has been studied using fluorescence line narrowing (FLN) and three-pulse photon echo peak shift (3PEPS). From the FLN measurement we extract the Huang-Rhys S-factors and frequencies of 16 Franck-Condon active vibrations clustered in two groups centred around 350 cm(-1) and 1400 cm(-1). The five modes below 500 cm(-1) covered by the excitation pulse spectral band create a complex beating pattern in the 3PEPS measurements. After determining the reorganisation energy of the modes involved in the peak shift measurement we are able to obtain the S-factors of the modes. The S-factors from two different experiments are in good agreement. The dephasing rates of these vibrations deviate significantly from the band-gap law. We propose that the main channel for the dephasing of the modes above 225 cm(-1) is a second order process involving one phonon and a molecular vibration. (C) 2010 Elsevier B.V. All rights reserved.
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24.
  • Christensson, Niklas, et al. (författare)
  • Four-wave-mixing spectroscopy of peridinin in solution and in the peridinin-chlorophyll-a protein
  • 2010
  • Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 373:1-2, s. 15-22
  • Tidskriftsartikel (refereegranskat)abstract
    • A model for the third order optical response of carotenoids is used to analyse transient grating and pump-probe data of peridinin in solution and bound in the peridinin-chlorophyll protein (PCP). For peridinin in solution, the transient grating signal detected at 505 nm exhibits a bi-exponential recovery whose fast phase is assigned to relaxation from the S-2 state that has a lifetime of 75 +/- 25 fs. The slower, solvent-dependent rise component is assigned to equilibration of the (S-1/ICT) state, taking place on a time scale of 0.6 and similar to 2.5 ps in acetontrile and benzene, respectively. These dynamics match those obtained from pump-probe measured in the spectral region of the ICT state, implying that the ICT state contributes to the signal at 505 nm. In PCP, the transient grating signal shows distinctly different kinetics, and the signal shows no recovery. This difference is explained by energy transfer from peridinin to chlorophyll-a. (C) 2009 Elsevier B. V. All rights reserved.
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25.
  • Christensson, Niklas, et al. (författare)
  • Origin of Long-Lived Coherences in Light-Harvesting Complexes
  • 2012
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 116:25, s. 7449-7454
  • Tidskriftsartikel (refereegranskat)abstract
    • A vibronic exciton model is applied to explain the long-lived oscillatory features in the two-dimensional (2D) electronic spectra of the Fenna-Matthews-Olson (FMO) complex. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations with dephasing times of 1.3 ps at 77 K, which is in a good agreement with the experimental results. These long-lived oscillations originate from the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. The oscillations obtain a large amplitude due to excitonic intensity borrowing, which gives transitions with strong vibronic character a significant intensity despite the small Huang-Rhys factor. Purely electronic coherences are found to decay on a 200 fs time scale.
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26.
  • Christensson, Niklas, et al. (författare)
  • Photon echo spectroscopy reveals structure-dynamics relationships in carotenoids
  • 2009
  • Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 79:24
  • Tidskriftsartikel (refereegranskat)abstract
    • Based on simultaneous analysis of the frequency-resolved transient grating, peak shift, and echo width signals, we present a model for the third-order optical response of carotenoids including population dynamics and system-bath interactions. Our frequency-resolved photon echo experiments show that the model needs to incorporate the excited-state absorption from both the S-2 and the S-1 states. We apply our model to analyze the experimental results on astaxanthin and lycopene, aiming to elucidate the relation between structure and system-bath interactions. Our analysis allows us to relate structural motifs to changes in the energy-gap correlation functions. We find that the terminal rings of astaxanthin lead to increased coupling between slow molecular motions and the electronic transition. We also find evidence for stronger coupling to higher frequency overdamped modes in astaxanthin, pointing to the importance of the functional groups in providing coupling to fluctuations influencing the dynamics in the passage through the conical intersection governing the S-2-S-1 relaxation.
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27.
  • Christensson, Niklas, et al. (författare)
  • Probing the strength of the system-bath interaction by three-pulse photon echoes.
  • 2009
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We explore how the width of the three-pulse photon echo signal at long population times can be used to determine the strength of the system-bath interaction. Using simulation with finite pulses we show that a simple relation exist between the width of the echo signal, the coupling strength, and the pulse autocorrelation. The derived model is applied to Rhodamine 6G in alcoholic solution, a paradigm system for the study of solvation dynamics, and the results are compared to conventional methods. The independently determined coupling strength forms the basis for a computationally inexpensive route to determine the entire spectral density, the key parameter when considering system-bath interactions. Our method allows us to accurately estimate the relative amplitude of fast and slow components in the correlation function using only impulsive limit simulations. We show that the peak shift significantly overestimates the amplitude of the fastest decay component for our experimental data. Changing solvent from methanol to 1-hexanol we observe a narrowing of the width of the echo profile. The changes in the echo width allow us to estimate the changes of the coupling strength in various solvents.
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28.
  • Christensson, Niklas, et al. (författare)
  • Solute specific polar solvation studied by photon echo spectroscopy
  • 2009
  • Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 357:1-3, s. 85-95
  • Tidskriftsartikel (refereegranskat)abstract
    • Polar solvation dynamics Of two Solute molecules in a series of 1-alcohols has been studied using the three-pulse photon echo peak shift technique. The inertial dynamics, on sub-200 fs timescale, is essentially independent of solvent in the 1-alcohol series. For a given solute, the solvent dependence is mainly manifested in the diffusive solvation dynamics. The solute dependence appears as a significantly stronger inertial component in one of the molecules. We ascribe this solute dependence to the differences in charge redistribution upon excitation. A detailed investigation of oscillations of the peak shift reveals a solvent dependent beating that can be connected to solvation of the vibrational mode in the excited state. The solvent dependence of the dephasing dynamics of the excited state mode can be explained by the electronic transition frequency correlation function and the beating pattern of the echo signal originates from interference between ground and excited state Feynman pathways. (C) 2008 Elsevier B.V. All rights reserved.
  •  
29.
  • Christensson, Niklas, et al. (författare)
  • Three-pulse photon echo peak shift in optically dense samples
  • 2008
  • Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 457:1-3, s. 106-109
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the dependence of the three-pulse photon echo peak-shift (3PEPS) signal on solute concentration and sample path length. The most prominent effect is an increase of the amplitude of the long decay component of the 3PEPS at high optical densities, but signal distortions are observed for all timescales in the experiments. The distortions of the peak shift signal are more severe for longer path lengths. These observations are assigned to pulse propagation effects. At low concentrations interference between the solute signal and the non-resonant solvent response severely distort the peak profiles at short population times.
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30.
  • Christensson, Niklas, et al. (författare)
  • Two-dimensional electronic spectroscopy of beta-carotene.
  • 2009
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 113:51, s. 16409-16419
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-dimensional electronic spectroscopy (2D) has been applied to beta-carotene in solution to shine new light on the ultrafast energy dissipation network in carotenoids. The ability of 2D to relieve spectral congestion provides new experimental grounds for resolving the rise of the excited state absorption signal between 18,000 and 19,000 cm(-1). In this spectral region, the pump-probe signals from ground state bleach and stimulated emission overlap strongly. Combined modeling of the time-evolution of 2D spectra as well as comparison to published pump-probe data allow us to draw conclusions on both the electronic structure of beta-carotene as well as the spectral densities giving rise to the observed optical lineshapes. To account for the experimental observations on all time scales, we need to include a transition in the visible spectral range from the first optically allowed excited state (S(2)-->S(n2)). We present data from frequency resolved transient grating and pump-probe experiments confirming the importance of this transition. Furthermore, we investigate the role and nature of the S* state, controversially debated in numerous previous studies. On the basis of the analysis of Feynman diagrams, we show that the properties of S*-related signals in chi(3) techniques like pump-probe and 2D can only be accounted for if S* is an excited electronic state. Against this background, we discuss a new interpretation of pump-deplete-probe and intensity-dependent pump-probe experiments.
  •  
31.
  • Christensson, Niklas, et al. (författare)
  • Weakly chirped pulses in frequency resolved coherent spectroscopy.
  • 2010
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 132:17, s. 1-174508
  • Tidskriftsartikel (refereegranskat)abstract
    • The role of weakly chirped pulses (time bandwidth product, DeltanuDeltatau<0.61) on three-pulse photon echo signals has been systematically studied. Pulses with varying chirp were characterized with frequency resolved optical gating (FROG) and used to measure spectrally resolved three-pulse photon echoes of a dye in solution. The weakly chirped pulses give rise to markedly different echo signals for population times below approximately 100 fs. The chirped pulses can decrease or enhance spectral signatures of an excited state absorption transition in the echo signal. Furthermore, the observed dephasing dynamics depend on the phase of the electric fields. Simulations based on a three-level model and the electric fields retrieved from the FROG traces give a good agreement for photon echo experiments with both transform limited and chirped pulses. The simulations also allow for a numerical investigation of effects of chirp in two-dimensional spectroscopy. For a two-level system, the chirped pulses result in nonelliptical two-dimensional spectra that can erroneously be interpreted as spectral heterogeneity with frequency dependent dephasing dynamics. Furthermore, chirped pulses can give rise to "false" cross peaks when strong vibrational modes are involved in the system-bath interaction.
  •  
32.
  • Dahlbom, Mats, et al. (författare)
  • Collective excitation dynamics and polaron formation in molecular aggregates
  • 2002
  • Ingår i: Chemical Physics Letters. - 0009-2614. ; 364:5-6, s. 556-561
  • Tidskriftsartikel (refereegranskat)abstract
    • Real-space collective excitation dynamics in molecular aggregates is studied using a model where the electronic system is described via exciton theory with surface hopping. The nuclear dynamics are included using the Langevin equation where temperature and zero-point motions are entered via the fluctuation-dissipation theorem. Dynamic processes like exciton relaxation, localization, polaron formation and diffusion of self-trapped excitons, which commonly require different theories, are simultaneously described with our approach. Numerical simulations of small linear aggregates are performed. Contrary to the common view we show that exciton relaxation can temporarily increase exciton delocalization. The results are discussed based on the photosynthetic light-harvesting pigment-protein complexes. (C) 2002 Elsevier Science B.V. All rights reserved.
  •  
33.
  •  
34.
  • Damtie, Fikeraddis A., et al. (författare)
  • Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots
  • 2016
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:6
  • Tidskriftsartikel (refereegranskat)abstract
    • Multiple exciton generation (MEG) is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.
  •  
35.
  • Dietzek, Benjamin, et al. (författare)
  • Appearance of intramolecular high-frequency vibrations in two-dimensional, time-integrated three-pulse photon echo data
  • 2007
  • Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084. ; 9:6, s. 701-710
  • Tidskriftsartikel (refereegranskat)abstract
    • An alternative experimental outline to measure homodyne detected three-pulse photon-echo data is presented. The novel experimental approach allowing for online monitoring and correction of experimental timing and stability is discussed in detail using the paradigm system of Nile blue in alcohol solution. It is shown that excellent signal-to-noise ratios together with high reproducibility of the data can be routinely achieved. We report in detail on the appearance of high-frequency intramolecular vibrations in the two-dimensional three-pulse photon-echo data and suggest that besides the conventionally discussed three-pulse photon-echo peak-shift the width of the integrated echo signal as a function of population time contains identical and easily accessible information on high-frequency intramolecular vibrations. A comparison of experimental data with theoretical modeling is performed showing that the observed echo-width oscillations are in line with predictions of the Brownian oscillator model.
  •  
36.
  • Dietzek, Benjamin, et al. (författare)
  • Ultrafast excited-state isomerization dynamics of 1,1 '-diethyl-2,2 '-cyanine studied by four-wave mixing spectroscopy
  • 2007
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 111:19, s. 5396-5404
  • Tidskriftsartikel (refereegranskat)abstract
    • Excited-state dynamics and solvent-solute interactions of 1,1'-diethyl-2,2'-cyanine iodine (1122C) in alcoholic solutions are investigated using time-integrated three-pulse photon-echo spectroscopy. 1122C serves as a model compound for ultrafast photoinduced isomerizationa key process in the light reception of plants, bacteria, and human vision. The photoreaction in 1122C is interrogated in dependence on solvent and excitation wavelength. The wavelength-dependent three-pulse photon-echo peak shift indicates strong alterations of the reaction pathways and points to the existence of a direct internal conversion channel in close proximity to the Franck-Condon point of absorption. The solvent-dependent S-1-S-0 internal conversion time does not follow conventional sheared viscosity dependence, suggesting that the solvent local friction has to be considered to account for the observed isomerization kinetics. The concerted discussion of transient grating and three-pulse photon-echo peak-shift data allows us to derive a complete picture of the solvent-solute interaction-controlled photoreaction. The results obtained are related to other work on reactive systems and are discussed in the framework of multilevel response functions.
  •  
37.
  • Eriksson, Axl, et al. (författare)
  • Synthesis of Well-Ordered Functionalized Silicon Microwires Using Displacement Talbot Lithography for Photocatalysis
  • 2024
  • Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 9:18, s. 20623-20628
  • Tidskriftsartikel (refereegranskat)abstract
    • Metal-assisted chemical etching (MACE) is a cheap and scalable method that is commonly used to obtain silicon nano- or microwires but lacks spatial control. Herein, we present a synthesis method for producing vertical and highly periodic silicon microwires, using displacement Talbot lithography before wet etching with MACE. The functionalized periodic silicon microwires show 65% higher PEC performance and 2.3 mA/cm2 higher net photocurrent at 0 V compared to functionalized, randomly distributed microwires obtained by conventional MACE at the same potentials.
  •  
38.
  • Finkelstein-Shapiro, Daniel, et al. (författare)
  • Coherent two-dimensional spectroscopy of a Fano model
  • 2016
  • Ingår i: Physical Review B. - 1098-0121. ; 94:20
  • Tidskriftsartikel (refereegranskat)abstract
    • The Fano line shape arises from the interference of two excitation pathways to reach a continuum. Its generality has resulted in a tremendous success in explaining the line shapes of many one-dimensional spectroscopies - absorption, emission, scattering, conductance, photofragmentation - applied to very varied systems - atoms, molecules, semiconductors, and metals. Unraveling a spectroscopy into a second dimension reveals the relationship between states in addition to decongesting the spectra. Femtosecond-resolved two-dimensional electronic spectroscopy (2DES) is a four-wave mixing technique that measures the time evolution of the populations and coherences of excited states. It has been applied extensively to the dynamics of photosynthetic units, and more recently to materials with extended band structures. In this paper, we solve the full time-dependent third-order response, measured in 2DES, of a Fano model and give the system parameters that become accessible.
  •  
39.
  • Fu, Siyuan, et al. (författare)
  • Generalized lock-in amplifier for precision measurement of high frequency signals
  • 2013
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 84:11
  • Tidskriftsartikel (refereegranskat)abstract
    • We herein formulate the concept of a generalized lock-in amplifier for the precision measurement of high frequency signals based on digital cavities. Accurate measurement of signals higher than 200 MHz using the generalized lock-in is demonstrated. The technique is compared with a traditional lock-in and its advantages and limitations are discussed. We also briefly point out how the generalized lock-in can be used for precision measurement of giga-hertz signals by using parallel processing of the digitized signals.
  •  
40.
  • Ghosh, Supriya, et al. (författare)
  • Ion Migration Heals Trapping Centers in CH3NH3PbBr3 Perovskite
  • 2017
  • Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 2:9, s. 2133-2139
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the local changes in photophysics at different microregions of a methylammonium lead bromide (MAPbBr3) perovskite crystal under illumination. Our results show that the emission from the structurally homogeneous region is blue-shifted compared to the emission from the inhomogeneous regions. The yield and spectrum of the emission from the structurally homogeneous region do not vary with the illumination time, whereas distinct light-induced changes are seen in the spectra from the inhomogeneous region. The changes in the spectra at long illumination time suggest that ion-migration inhibits the emission from the inhomogeneous regions. The measurements of the emission lifetime suggest that the emission from the inhomogeneous regions is dominated by the defect-related emission at short illumination times and the band-to-band emission at the longer illumination times. Our work provides direct evidence for the light-induced healing of the defect centers, which is important in the design of photoactive devices of MAPbBr3.
  •  
41.
  • Grage, Mette, et al. (författare)
  • Conformational Disorder and Energy Migration in MEH-PPV with Partially Broken Conjugation.
  • 2003
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606. ; 118:16, s. 7644-7650
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo–simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV. ©2003 American Institute of Physics.
  •  
42.
  • Grage, Mette, et al. (författare)
  • Ultrafast Excitation Transfer and Trapping in a Thin Polymer Film.
  • 2003
  • Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 67:20, s. 1-205207
  • Tidskriftsartikel (refereegranskat)abstract
    • Transient absorption anisotropy of a polythiophene polymer in a thin film was studied on a femtosecond time scale. The anisotropy has a non-exponential decay on the sub-picosecond time scale, with a fastest component characterized by an ∼40 fs time constant. To simulate the anisotropy decay an incoherent energy migration model has been used. Comparison between the simulated and experimental kinetics enabled us to estimate the nearest-neighbor pair wise hopping time (τh=1±0.1 ps), the fraction of the interchain aggregates (∼10%) and the structural disorder of the polymer. The initial ∼30 fs anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit.
  •  
43.
  • Guo, Ruiqi, et al. (författare)
  • CuInSe2 Quantum Dots Hybrid Hole Transfer Layer for Halide Perovskite Photodetectors
  • 2018
  • Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 10:41, s. 35656-35663
  • Tidskriftsartikel (refereegranskat)abstract
    • A novel hybrid hole transport layer (HTL) of CuInSe2 quantum dots (QDs)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was developed to enhance the performance of halide metal perovskite (MAPbI3)-based photodetectors. The introduction of CuInSe2 QDs not only improved the wettability of the PEDOT:PSS HTL for the growth of perovskite crystals but also facilitated the transportation of holes from the perovskite to the HTL. As a result, both responsivity and detectivity of the device were increased dramatically by CuInSe2 QDs hybrid HTL, showing excellent photoresponsivity of 240 mA/W, larger ratio of photocurrent density to dark current density of 4.1 × 106, fast on-off switching properties of <0.02 s, and remarkable detectivity values of 1.02 × 1013 Jones at 580 nm and above 5.01 × 1012 Jones over the visible light region without an external bias voltage. In addition, the photodetectors also showed excellent thermal stability in the range of 10-110 °C. Therefore, a unique design idea of a hole transport material would be an anticipated direction for efficient halide metal perovskite-based devices.
  •  
44.
  • Guo, Ruiqi, et al. (författare)
  • Manganese doped eco-friendly CuInSe2 colloidal quantum dots for boosting near-infrared photodetection performance
  • 2021
  • Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 403
  • Tidskriftsartikel (refereegranskat)abstract
    • CuInSe2 (CISe) colloidal quantum dots (QDs) display promising applications in photodetection especially within near-infrared (NIR) regions due to their high extinction coefficient and environmental-friendly. However, the high trap density and poor crystal quality introduced by the ternary structure result in low photodetection of CISe QDs devices. Herein, we dope transition metal manganese ions (Mn2+) into CISe QDs to tackle the above problems. Structural characterization results demonstrate the crystal quality of CISe QDs is improved by doping Mn2+ during the synthesis of QDs. The transient absorption (TA) spectroscopic study together with the space-charge-limited current (SCLC) measurements show the charge carrier lifetime of Mn-CISe QDs is much longer than that of the CISe QDs, due to the Mn2+ doping state serve as hole capturer forming a charge-compensated pair with the Cu2+ defect that makes the long-lived Cu2+ radiative recombination dominate. Furthermore, Mn2+ doping concurrently modifies the conduction band minimum and valence band maximum level of the QDs verified by the ultraviolet photoelectron spectroscopy (UPS), which determines the driving force for charge carrier transfer to acceptors. The optimal Mn2+ doping level (0.01 Mn:Cu feed ratio) balanced the above two factors in the QDs. The detector based on such Mn-CISe QDs exhibits responsivity of 30 mA/W and specific detectivity of 4.2 × 1012 Jones at near-infrared wavelength, the response speed of 0.76 µs, and suppressed dark current density of 1.6 × 10−10 A cm−2.
  •  
45.
  • Hansen, Thorsten, et al. (författare)
  • Nonlinear response theory on the Keldysh contour
  • 2012
  • Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 45:15
  • Tidskriftsartikel (refereegranskat)abstract
    • Following the work of Schwinger (1961 J. Math. Phys 2 407), we present a general method for deriving quantum response functions on a closed-loop contour in the complex time plane. We focus on optical response functions of linear to third order and demonstrate by projecting contour time onto the real axis how they connect to Liouville space pathways. This work highlights the close connection between the Keldysh contour and the double-sided Feynman diagrams used in nonlinear optical spectroscopy. In addition, we give a Keldysh contour derivation of the Marcus equation for electron transfer.
  •  
46.
  • Hansen, Thorsten, et al. (författare)
  • Orbital Topology Controlling Charge Injection in Quantum-Dot-Sensitized Solar Cells
  • 2014
  • Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:7, s. 1157-1162
  • Tidskriftsartikel (refereegranskat)abstract
    • Quantum-dot-sensitized solar cells are emerging as a promising development of dye-sensitized solar cells, where photostable semiconductor quantum dots replace molecular dyes. Upon photoexcitation of a quantum dot, an electron is transferred to a high-band-gap metal oxide. Swift electron transfer is crucial to ensure a high overall efficiency of the solar cell. Using femtosecond time-resolved spectroscopy, we find the rate of electron transfer to be surprisingly sensitive to the chemical structure of the linker molecules that attach the quantum dots to the metal oxide. A rectangular barrier model is unable to capture the observed variation. Applying bridge-mediated electron-transfer theory, we find that the electron-transfer rates depend on the topology of the frontier orbital of the molecular linker. This promises the capability of fine tuning the electron-transfer rates by rational design of the linker molecules.
  •  
47.
  • He, Yanmei, et al. (författare)
  • Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals
  • 2022
  • Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 7:44, s. 39970-39974
  • Tidskriftsartikel (refereegranskat)abstract
    • Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr3) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr3microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.
  •  
48.
  • He, Yanmei, et al. (författare)
  • Nature of Self-Trapped Exciton Emission in Zero-Dimensional Cs2ZrCl6 Perovskite Nanocrystals
  • 2023
  • Ingår i: The Journal of Physical Chemistry Letters. - 1948-7185. ; 14:34, s. 7665-7671
  • Tidskriftsartikel (refereegranskat)abstract
    • Low dimensional perovskite-inspired materials with self-tapped exciton (STE) emission have stimulated a surge of cutting-edge research in optoelectronics. Despite numerous efforts on developing versatile low-dimensional perovskite-inspired materials with efficient STE emissions, there is little emphasis on the intrinsic dynamics of STE-based broad emission in these materials. Here, we investigated the excited state dynamics in zero-dimensional (0D) Cs2ZrCl6 nanocrystals (NCs) with efficient blue STE emission. By using femtosecond transient absorption (fs-TA) spectroscopy, the ultrafast STE formation process within 400 fs is directly observed. Then, the formed STEs relax to an intermediate STE state with a lifetime of ∼180 ps before reaching the emissive STE state with a lifetime of ∼15 μs. Our work offers a comprehensive and precise dynamic picture of STE emission in low-dimensional metal halides and sheds light on extending their potential applications.
  •  
49.
  • He, Zhi, et al. (författare)
  • Influence of the protein binding site on the excited states of bacteriochlorophyll: DFT calculations of B800 in LH2
  • 2002
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:44, s. 11606-11612
  • Tidskriftsartikel (refereegranskat)abstract
    • Effects of hydrogen bonding and the axial ligand interaction on the B800 band in two LH2 complexes Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum have been theo retically investigated by using density functional theory. The local electrostatic environment of the B800 bacteriochlorophyll is simulated as an atomic charge field consisting of the pigments in the protomer unit. Despite the fact that the B800 binding sites in two structures are very different, their absorption spectra are almost identical. Our calculations indicate that the charged axial ligand in Rs. molischianum and the hydrogen bonding in Rps. acidophila lead to similar red shifts, possibly explaining the above controversy. We also found (i) additional B800 bacteriochlorophyll transitions located between the Q and Soret regions for both LH2 complexes and (ii) the ligand to the B800 charge-transfer excited states in the long-wavelength region for the B800-alphaAsp(6) complex in,the Rs. molischianum LH2 system.
  •  
50.
  • Herek, Jennifer, et al. (författare)
  • Ultrafast carotenoid band shifts: Experiment and theory
  • 2004
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:29, s. 10398-10403
  • Tidskriftsartikel (refereegranskat)abstract
    • The ultrafast carotenoid band shift upon excitation of nearby bacteriochlorophyll molecules was studied in three different light harvesting complexes from purple bacteria. The results were analyzed in terms of changes in local electric field of the carotenoids. Time dependent density functional theory calculations based on known and model structures led to good agreement with experimental results, strongly suggesting that the mutual orientation of the pigment molecules rather than the type of the carotenoid molecules determines the extent of the ultrafast band shift. We further estimate that the protein induced local field nearby carotenoid molecule is about 4 or 6 MV/cm, depending on the orientation of the change of the electrical dipole in the carotenoid upon optical transition.
  •  
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